This invention relates to pipe-laying vessels and to methods of laying a pipeline. Such vessels and methods are of particular use in the offshore industry relating to oil and gas production.
One of the tasks that is carried out by certain vessels in the offshore industry is that of laying pipelines, either by S-laying or by J-laying.
In “S” laying, the pipeline leaves the vessel at little or no inclination to the horizontal, adopts a steeper inclination in the water and then returns to a generally horizontal disposition on the seabed. The tension in the pipeline is often accommodated by a series of track tensioners mounted along the pipe-laying path, on the vessel. The track tensioners support the weight of the pipeline and control passage of the pipeline. S laying is preferred in shallower water where the natural path of the pipeline is only ever inclined at a shallow angle as it passes to the seabed. Examples of S-laying arrangements are shown in WO 2006/085739 and WO 2007/000609.
In “J” laying, the pipeline leaves the vessel at a steep or vertical inclination and the inclination steadily reduces until the pipeline is in a generally horizontal disposition on the seabed. J laying often involves moving a new pipeline section from a substantially horizontal position (along the deck of the vessel) into a vertical position to align with a J lay tower mounted on the vessel. A lower end of the new pipeline section is welded to the upper end of the pipeline, which is held in place, suspended from the vessel, by a fixed clamp, located towards the bottom of the J lay tower. Once the new section of pipeline has been added to the existing pipeline, the fixed clamp is released and the pipeline lowered down the J lay tower. During such lowering the tension in the pipeline may be accommodated by a travelling block or track tensioners. The upper end of the newly lengthened pipeline (i.e. the upper end of the new section of pipeline) is then clamped by the fixed clamp, the travelling block, if used, is returned to its original position towards the top of the J lay tower and the process repeated. J-laying is used primarily for laying pipeline in deep water (that is typically deeper than 1,000 m). Examples of J-laying arrangements are shown in WO 2009/153352 and WO 2009/153354.
It is an object of the invention to provide an improved pipe-laying vessel and an improved method of laying a pipeline.